1. Field of the Invention
The invention relates to a lubricating apparatus and method for lubricating a string of hollow rods connected to a cone penetrometer as used in geotechnical "in situ" testing.
2. Prior Art
Cone penetrometers have been used for several decades for assessing properties of penetrable ground such as sand, silt, and other relatively soft overburden that can be penetrated without using drilling techniques. There are two main types of cone penetrometers, namely a dynamic type using "standard" hammer blows, and a static type using a steady push to produce a constant rate of penetration. The static type has been modernized and an electric type is available. The electric cone penetrometer, which is also known as the Dutch cone or the Fugro cone, has an elongated cylindrical body, about 1.5 meters long, and has a conical outer end containing transducers for measuring properties of the overburden such as end bearing loads or penetration resistance, skin friction, and pore pressure. The transducers provide signals which are transmitted either as electrical signals along wires in aligned bores of the rods, or as ultrasonic signals along the string of rods to a recording device positioned usually above the surface.
The static cone is pushed through the overburden using a force generator, such as a hydraulic ram system, which is designed to produce an essentially constant rate of penetration through the overburden to produce a probe hole. The cone penetrometer can have an enlarged portion which has a diameter slightly greater than that of the rods pushing the penetrometer, to reduce affect of the rods on the penetration force. However, usually the side wall of the probe hole moves inwards and the rods buckle slightly under the force and can drag on the side wall or inside of the probe hole, and the resulting drag limits depth of penetration of the cone penetrometer, usually to about thirteen to fifteen meters below the surface. This limit is reached in relatively soft ground, when using a ten ton deadweight as a reaction to the force generator and clearly a shallower limit is reached in overburden that is more difficult to penetrate. It is considered that this limitation of penetration is almost entirely due to drag between the rods and the sides of the probe hole, which drag clearly increases as the length of rods increase. The drag is a combination of friction and lateral forces acting in the rod. Similarly a limit due to drag of the rods on the probe hole is reached when using the dynamic type of cone penetrometer.